The present invention relates to circuits for adjusting the thermal response of a bridge circuit.
One type of transducer changes its resistance in response to external stimuli. For instance, a pressure transducer will vary its resistance in proportion to the amount of pressure exerted on the transducer. One type of pressure transducer has four legs forming a bridge, with two of the legs increasing in resistance with pressure and the other two legs decreasing in resistance with pressure. The value of the resistance will vary with temperature, and accordingly a temperature-variable resistor (thermistor) is typically added to the circuit to offset the variation in resistance of the bridge with temperature.
After the addition of the thermistor, minor changes to the resistance of the bridge are still often necessary. This is typically done using trimming resistors. The trimming resistors are coupled to the bridge circuit and are typically a resistive area film which is cut with a laser to decrease its area and thus trim the resistance value.
Two types of changes with temperature are typically compensated for. One such change is referred to as a change in "span". For example, in a differential pressure transducer, the range (span) of the output voltage (minimum and maximum voltage) will vary with temperature. Thus, the span will require compensation. The other characteristic which varies is referred to as the "zero" value, which is the output voltage at a reference temperature and pressure. Other methods for performing zero and span compensation involve modifying the output signal from the transducer according to an algorithm to compensate for the temperature variations. The following patents discuss typical methods and circuits used.
Mergner U.S. Pat. No. 4,478,527 uses temperature variable resistors to create a temperature slope, a span slope and a DC offset. The resultant signals are then summed.
Brown U.S. Pat. No. 4,209,837 shows a switched current source.
Cooper U.S. Pat. No. 4,667,143 discloses a closed loop sensor which varies a voltage as a function of temperature using a switched mode integrated circuit.
Schorn Protective Disclosure No. T943003 provides a method of selecting a predetermined combination of fixed resistors and thermistors.
Rao U.S. Pat. No. 3,882,725 shows a circuit for compensating a single variable leg bridge for temperature and initial zero.
Wilbur U.S. Pat. No. 3,842,674 shows a variable potentiometer for controlling zero and span.
Horinouchi U.S. Pat. No. 4,484,050 increases the accuracy of a thermistor by straightening the thermistor curve and digitally sets a reference voltage.
Momin U.S. Pat. No. 4,628,316 discloses a method for performing an error calculation for temperature variations.
Takamatsu U.S. Pat. No. 4,462,003 discloses a method of selecting a set of fixed resistors to program gain.
Miles U.S. Pat. No. 4,523,161 discusses a method for selecting a set of fixed resistors to program attenuation.
Townzen U.S. Pat. No. 4,481,596 shows a method for summing two curves to cancel the errors.
Wiloner U.S. Pat. No. 4,232,269 uses switched resistors to program a filter response.